I'm sure Ren is right;
glass is here a word for a barometer, and is still used by sailors. I've done a search on Google for examples, and came up with the following words of wisdom
here:
The Barometer
Long foretold, long last,
Short notice, soon past,
Quick rise after low,
Sure sign of stronger blow.
When the glass falls low,
Prepare for a blow;
When it slowly rises high,
Lofty canvas you may fly.
At sea with low and falling glass
Soundly sleeps a careless ass,
Only when it's high and rising
Truly rests a careful wise one.
Before embarking on your solo circumnavigation of the globe, I'll add their words of warning
"Although there is a considerable degree of truth in them, these rhymes are not infallible and you should use them with caution".